Those well versed in the pulp and paper making industry are well acquainted with the term “white liquor”. It is long been known that pulp yield in the pulp and paper manufacturing processing may be increased substantially by forming polysulfide in the white liquor. Many attempts have successfully been made to take advantage of the yield improvement of polysulfide. However, in the course of these developments, problems have been encountered with the introduction of manganese dioxide into the white liquor to the point that although yield improvements have been obtained, the return on investment has been insufficient to justify the cost of ridding the process of problems caused by the presence of manganese dioxide.
In a typical process, a slurry combination of manganese dioxide and calcium carbonate, usually referred to as sludge, is combined with the white liquor. The manganese dioxide reacts with sodium sulfide in the mixed liquor to cause the formation of polysulfide and a yield improvement results.
However, if too much manganese dioxide is injected into the white liquor stream, it may pass through the entire system and create black specks in the resulting paper which appear as color contaminates, making the paper unsuitable for many uses. As a consequence the color contaminated paper cannot fetch as high a price on the market as it would without the contamination so the yield savings are lost as a result of the price reduction.
It has been determined that filtration could be used to remove excess manganese dioxide. However, the filters required are expensive. Further, filtration would necessarily require that the calcium carbonate be filtered out prior to the filtration to recapture the manganese dioxide. In short, two filters would have to be employed and the capital expense of the equipment for the system is considerably increased, negating the benefit of the yield advantage provided by polysulfide to the point where a favorable return on investment cannot be obtained.
The present invention is directed to overcoming one or more of the above problems.